I own the porta-pros, KSC35, KSC50, The Plug and a radio-shack version of the Koss Pro35 (which I got for $20 on sale locally).
The Porta/sporta (same diff) are boomier and louder than the 35's because they are tighter against the head. The 35's have a cleaner sound, but weaker bass, so I drive them with a little EQ on the bass side.
The ksc50's, which fit on the ear like the ksc35, are looser and have even less bass and almost a tinny sound.
The RS Pro-35, which are traditional open / over the head band phones, have the best sound of them all for me. I think the ksc35, portas, and RS pro-35 all have the same speaker/driver, but the fit makes for different sounds. Though, the RS pro35 list that they have a Titanium driver (not sure if the KOSS phones have Ti drivers). The fit makes a diff and thus, the RS pro 35 have the best fit--not too tight, not too loose.
One mod I performed on my portas that seemed to help the boomy-muddiness, was to cut out a small dime-sized hole in the center of the pad, revealing the center of the speaker. I haven't performed this mod on the pro-35 or ksc35, since they sound good as is.
The latest version of the radio shack pro-35 phones (item 33-1222) can be found:http://www.radioshack.com/product.as...5Fid=33%2D1222
PS. The diff between the portas/RS-35/KSC35 is not that huge. they're all very similar sounding, and no matter which you get, you'll be glad you did. Go for the fit you want most.